Differential amplifiers are designed to selectively amplify the difference between low-level electrical signals while rejecting large common-mode signals. Dc-coupled differential amplifier designs, such as instrumentation amplifiers, achieve large gain, high input impedance, and high common-mode rejection ratio (CMRR). This invention relates to differential amplifiers including those used as ac-coupled instrumentation amplifiers.
Although instrumentation amplifier designs offer superior signal processing performance for dc-coupled measurement applications, much of the performance is degraded by the modifications presently used to design ac-coupled differential amplifiers. Optimization of instrumentation amplifier designs for specific signal processing applications has been described in numerous technical journals and textbooks. For example, see sections 7.09 and 7.10 (pp. 421-428) of The Art of Electronics, Second Edition, P. Horowitz and W. Hill, Cambridge University Press, 1990. Given an appropriate dc-coupled differential or instrumentation amplifier design, two general approaches have been used to design ac-coupled differential amplifiers: (1) directly ac-couple both inputs of a dc-coupled instrumentation or differential amplifier (i.e., connect RC high pass filters across the inputs) or (2) connect ac-coupled, unity-gain buffer amplifiers to each input of a dc-coupled instrumentation or differential amplifier. Both approaches presently used for designing ac-coupled differential amplifiers produce amplifiers with significant limitations. The first approach offers less complexity than other alternatives; however, directly ac-coupling an instrumentation or differential amplifier loads the input of the amplifier, which substantially lowers input impedance of the differential amplifier and degrades CMRR of the differential amplifier (particularly at frequencies over a hundred hertz). The second approach produces an ac-coupled differential amplifier with very high input impedance; however, ac mismatches between the input buffer amplifiers substantially degrade CMRR of the differential amplifier, particularly at frequencies over a few hundred hertz.
To overcome the difficulties associated with designing ac-coupled differential amplifiers with performance adequate for use in a wide range of measurement applications, numerous specific designs have been developed to optimize performance of ac-coupled differential amplifiers for specific applications with particular processing requirements. For example, in U.S. Pat. No. 4,320,351, issued to Brown, the differential amplifier represents an example of the second general design approach described above. Specifically, two high-input-impedance, ac-coupled buffer amplifiers were connected to a differential amplifier in a fashion that produced an ac-coupled differential amplifier with superior performance over a very narrow range of frequencies (approximately 10 to 100 Hz). Notice in FIG. 2 of U.S. Pat. No. 4,320,351 that CMRR of the amplifier degrades at very low frequencies (greater than 10 Hz); as discussed above, the degraded CMRR occurs due to ac mismatches between the buffer amplifiers connected to both inputs of the differential amplifier. In general, all prior ac-coupled differential amplifiers have been designed to insure sufficient performance for specific applications.
No prior ac-coupled differential amplifier design has been able to retain all of the superior performance, including high input impedance and high CMRR, inherent in dc-coupled instrumentation and differential amplifier designs.
A need thus exists for an ac-coupled differential amplifier design that retains all of the superior performance inherent in dc-coupled instrumentation and differential amplifier designs. A need also exists for an ac-coupled differential amplifier design that simultaneously offers high input impedance and high CMRR. A further need exists for an ac-coupled differential amplifier design that can achieve superior performance with substantially less complexity than prior designs. A need also exists for an ac-coupled differential amplifier design that can achieve superior and reliable performance at substantially less cost than prior designs. Finally, a need exists for an ac-coupled differential amplifier design that can be produced by simple and inexpensive modifications of many commonly used instrumentation and differential amplifier designs.